1. Field of the Invention
The present invention relates to an optical wireless transmission apparatus which transmits/receives a light signal modulated by a data signal to transmit data.
2. Description of the Related Art
In an indoor optical wireless system in which a conventional optical wireless transmission apparatus is used, there has been a mainstream that one apparatus constituting the system automatically finds a target apparatus to communicate with the target apparatus. In this system, the apparatus receives light emitted from the target apparatus (hereinafter referred to as a pilot light) and detects a direction of the light to emit a transmission light in the direction. For example, as shown in FIG. 1, one apparatus (main apparatus 21) is provided with light emitting means 23 separately from a light emitting section 22 for transmitting a data signal, and the light emitting means 23 sends a pilot light 23A for optical axis adjustment. The other optical wireless transmission apparatus (sub-apparatus 24) displaces an optical axis direction of a light receiving device 24A to receive the pilot light 23A, and optical axes are adjusted based on a received light level of the pilot light 23A (see Japanese Patent No. 3059870).
Additionally, as a light for obstructing the pilot light sent for the optical axis adjustment, there are illuminations such as a fluorescent light. As a countermeasure against the fluorescent light, a filter for cutting a visible light is disposed, a modulated light is used as the pilot light, and a reception light is received at a frequency of the pilot light.
Moreover, there is also sunlight as the light which obstructs the pilot light. In general, a light in a near-infrared region, having a wavelength of around 850 nm, is used as the pilot light, but the sunlight is incomparably stronger than the light of the near-infrared region as compared with the fluorescent light. Therefore, when only the filter for cutting the visible light is disposed, the countermeasure against the sunlight is insufficient. The pilot light is easily obstructed by the sunlight in a situation where the sunlight is directly incident upon the inside of a room, and there has been a problem that it is difficult to exactly detect a position of the target apparatus from the pilot light emitted from the target apparatus.